The resistance of copper (Cu) lines increases as scaling continues in the back-end-of-line (BEOL). Resistance increases first simply because the lines are having a smaller cross sectional area. Also contributing to the overall resistance, as metal surfaces in a line become closer and grains in the metal become smaller in narrower lines, an increase in scattering from grain boundaries and surfaces/interfaces further contribute to an additional increase in resistivity, thus highlighting the importance of controlling interface and grain boundaries scattering in scaled Cu line technology.
Concurrently, other problems arise as scaling continues. One problem is diffusion barrier integrity. Namely, as liner thickness is reduced, performance is not sufficient and metals can diffuse into the dielectric. Since liner thickness needs to be maintained, the volume of Cu in very narrow lines is limited. Another problem is electromigration or EM resistance. Namely, as line area is reduced, the current density increases. As a result, Cu atom migration can lead to breaks in the smallest lines. Therefore, defining good EM interfaces is important. Good bounded interface will also likely help for interface scattering. There is also a need for a cap to prevent EM at the top surface of the line. Yet another problem that arises with Cu line scaling is adhesion at the bottom of the vias. Good adhesion is required to prevent crack propagation during chip processing. A reduction in liner thickness at the bottom of vias does not help adhesion between levels.
Proposed solutions have included use of self-forming embedded diffusion barriers containing materials such as manganese (Mn), cobalt (Co) and/or ruthenium (Ru). See, for example, U.S. Pat. No. 9,190,321 issued to Cabral Jr. et al., entitled “Self-Forming Embedded Diffusion Barriers.” See also U.S. Patent Application Publication Number 2011/0045171 by McFeely et al., entitled “Multi-Step Method to Selectively Deposit Ruthenium Layers of Arbitrary Thickness on Copper” and U.S. Pat. No. 7,884,475 issued to Gambino et al., entitled “Conductor Structure including Manganese Oxide Capping Layer.” All of these solutions involve elements that are either fully soluble in Cu (e.g., Mn) or have very low solubility with no possible phase forming (e.g., Co, Ru, etc.)
Therefore, improved barriers, liners, and caps for Cu interconnects are needed especially for scaled applications.